Abstract : Results of theoretical calculations on the characteristics of the optically pumped rubidium 87 maser are given. It is shown that the maser power output is a critical function of the rubidium density, light intensity and cavity Q. Fundamental parameters and constants of the maser are grouped together to define an oscillation parameter gamma m prime which determines the oscillation characteristics of the maser. Spin exchange calculations in rubidium are presented. The relaxation times T sub 1 and T sub 2 due to spin exchange interactions are related to each other and it is shown that T sub 2 approximately equals 8/5 T sub 1. Experiments intended to provide a better understanding of the basic principles underlying the operation of the maser are described. These experiments consist of relaxation time measurements of rubidium in buffer gases such as nitrogen and neon. It is found that an optimum pressure exists at which the relaxation times are longest; this pressure is not the same for T sub 1 and T sub 2. Maximum signal was observed in nitrogen at a pressure of approximately 10 Torr. This result is in agreement with published data. Experimental results are given on relaxation of rubidium through spin exchange. It is found that the experimental data are in agreement with the theoretical calculations. A method of determining the spin exchange cross-section is described; in this method the cross-section is obtained in terms of fundamental or measurable parameters of the maser and it does not require a knowledge of the rubidium density.